The present invention relates to an apparatus and a method for receiving signals modulated by a multi-carrier modulation scheme for transmitting the information code using a plurality of carriers, or in particular to a receiving apparatus for receiving the signals containing data carriers and regularly-inserted pilot carriers having a fixed amplitude and a fixed phase.
This invention is suitably applicable to a receiving apparatus for receiving the signals modulated by the OFDM (Orthogonal Frequency Division Multiplex) modulation scheme with pilot carriers inserted therein, and an OFDM receiving apparatus having the functions of clock synchronization or generation of a delay profile utilizing the correlation of guard intervals.
In recent years, the OFDM modulation scheme, which is highly resistant to multi-path fading and ghost, has been closely watched as a modulation scheme suitable for application to the digital audio broadcasting for mobile units and the terrestrial digital TV broadcasting. The OFDM modulation scheme is a type of multi-carrier modulation scheme in which n (n: several tens to several hundreds) carriers orthogonal to each other are digitally modulated.
The I-axis component and the Q-axis component of each of these carriers is assigned a discrete code as a modulated signal, which code is updated every symbol period (several tens of xcexcs). As shown in FIG. 2, a multiplicity of digitally modulated waves xcex94f, 2xcex94f, 3xcex94f, . . . , (nxe2x88x921)xcex94f are added to each other, and an OFDM signal obtained by orthogonal modulation of the I and Q axes is transmitted. The multi-carrier digital modulation schemes often used are multi-valued modulation schemes such as the 16 quadrature amplitude modulation (16QAM) and the 32 quadrature amplitude modulation (32QAM) as well as the differential quadrature phase shift keying (DQPSK).
The OFDM signal is obtained by subjecting a plurality of the carriers of the I and Q components to the inverse fast Fourier transform (IFFT) and converting the carriers into a temporal waveform. The OFDM signal is configured, as shown in FIG. 3, of an effective symbol providing a temporal waveform after the IFFT processing and a guard interval copied from a part of the valid symbol and added before the valid symbol. According to the OFDM modulation scheme, the addition of the guard interval can prevent the deterioration due to the inter-symbol interference of the delayed wave in the delay time during the guard interval period. Therefore, the OFDM modulation scheme has a high resistance against the multi-path fading. In this way, the signal identical with the end portion of the symbol is added to the head of the symbol as a guard interval, and therefore, in the OFDM transmission apparatus, the cross-correlation between the head and the end of the symbol is utilized for demodulation and synchronization.
Next, the processing of the received signal utilizing the correlation of the guard intervals is explained.
In the OFDM modulation scheme, the frequency intervals between subcarriers are so small that the interference between subcarriers is likely to occur due to the carrier frequency error between the transmitter and the receiver and the sampling clock frequency error of the demodulation system. The reproduction of these frequencies, therefore, requires a high accuracy. Specifically, in order to continue to receive the OFDM signal correctly, the sampling clock reproduction process is required in which the sampling clock frequency at the receiving end is kept coincident with the sampling clock frequency of the transmission signal at highly accuracy.
In view of this, the technique for reproducing the sampling clock utilizing the correlation of the guard intervals is disclosed in U.S. Pat. No. 5,602,835. An OFDM receiving apparatus utilizing the correlation of the guard intervals is briefly explained with reference to FIG. 16.
The received signal input to the receiving antenna 1 is converted to a baseband signal through a high-frequency unit 2 and an intermediate-frequency unit 3. The baseband signal is converted from an analog form to a digital form by an A/D converter 4. The received sampling series (R) converted to digital form are returned by a FFT (fast Fourier transform) operation unit 8 to a frequency waveform signal from a temporal waveform signal subjected to the IFFT operation at the transmitting end, and demodulated to the original signal by a demodulation unit 9.
The receiving sampling series (R) and the signal delayed by a time length equal to the valid symbol period in a valid symbol delay unit 11 are subjected to the cross-correlating process to each other by a correlator 13. From the result of this cross-correlating operation, only the result of the correlating operation for the signal range corresponding to the guard interval period is extracted. Then, the range of extraction of the correlating operation result is shifted by one sample period and the corresponding correlating operation result is extracted. Over the entire signal section for the cross-correlating operation, the same extraction process is repeated while shifting the extraction range of the correlating operation result by one sample period each time. The values of the repeated correlating operation thus extracted are added to obtain a waveform having a peak indicating the position of a guard interval as shown in FIG. 4. Outside the guard interval period, the two signals have no correlation and, therefore, the correlation result is approximately zero in value. With the arrival at the guard interval, however, the correlation begins to present itself and the result of addition value increases linearly. The correlation value reaches a peak at the time point when the addition period coincides with the guard interval. After this time point, the addition period begins to be displaced from the guard interval, and therefore the correlation value begins to decrease. The correlation waveform, therefore, assumes a triangle as shown in FIG. 4. Actually, however, this triangular wave is usually contained noises due to the random characteristic of the OFDM signal as described later.
The peak of the triangular wave representing the correlation value indicates the ending time point of the guard interval. In the symbol timing detector 5 for detecting the peak position, therefore, the sampling clock frequency of a VCO 7 is controlled through a VCO controller 6 in such a manner as to maintain the peak at a predetermined position. In this way, the clock reproduction processing can be realized.
The OFDM transmission, for the feature of the modulation scheme thereof, is often used for the mobile communication. In outdoor transmission, a multiple communication paths are formed to transmit the main wave arriving directly from the transmitter and waves reflected on buildings and the like and arriving with a delay time in accordance with the topographic conditions. Further, in mobile communication, a fading environment may be generated with the levels of the main wave and the reflected waves changing each moment. The multi-path communication undesirably causes the synchronization of the FFT demodulation windows at the receiving end with the reflected waves or the inter-symbol interferences in the presence of reflected waves exceeding the range of the guard intervals.
This inter-symbol interference causes a deterioration of the carrier-to-noise ratio (C/N), and causes the deterioration of the code error rate. In order to improve the reliability of the transmission, therefore, it is necessary to select a propagation path environment free of reflected waves exceeding the guard intervals. Very effective means for improving the reliability of communication, therefore, is to observe the propagation path characteristics and to employ a transmission scheme suited to the particular propagation path characteristics.
The most widely used method for observing the propagation path characteristics is the measurement of a delay profile indicating the levels of the main wave and the reflected waves and the delay time, as disclosed in JP-A-2002-232389, for example. In order to produce this delay profile, the received sampling signal and a signal obtained by delaying the received sampling signal by the length equivalent to the valid symbol period are cross-correlated by the correlator 13 shown in FIG. 16, and the result of correlation is averaged in the direction of the symbol (along the time axis) thereby to obtain a waveform as shown in FIG. 5.
The result of averaging the correlated values is processed by a differentiator 10 shown in FIG. 16, thereby making it possible to calculate a differential waveform as shown in (a) of FIG. 6. Further, the negative signal components of the differential waveform are removed. In this way, a delay profile waveform as shown in (b) of FIG. 6 is obtained.
As described above, according to the OFDM scheme, each carrier may be digitally modulated by a scheme such as 16QAM or 32QAM. In these modulation schemes, the carrier signal is mapped to the information code in constellations as indicated by black spots in FIG. 7, so that the information is contained in the amplitude (distance from the origin) and the phase (angle from a reference point) of each carrier. For demodulation, therefore, the absolute amplitude and the absolute phase of each received carrier are required to be reproduced accurately.
In the adverse environments of the mobile communication and the multi-path communication, the amplitude and phase of each carrier undergoes a change at each moment. The amplitude and phase, therefore, must be reproduced sequentially at high speed. For this reason, pilot carriers having a constant amplitude and a constant phase are arranged for each several carriers of the transmission signal, and based on the pilot carriers of the received signal, the propagation path characteristics are estimated. In accordance with the estimation result, a proper method for reproduction of the amplitude and the phase is selected. Also, in order to improve the transmission performance, the amplitude of the pilot carrier is often set to a somewhat larger value than that of the normal data carrier. For example, the amplitude of the pilot carrier is set to about {fraction (4/3)} of the amplitude of the data carrier.
The pilot carriers are arranged differently depending on the system conditions. An example of pilot carrier arrangement is described with reference to FIGS. 8A and 8B.
In FIG. 8A, the pilot carriers are arranged at intervals of eight carriers, i.e. for each symbol. These pilot carriers are arranged continuously along the direction of the symbol (along the time axis) and therefore are called continuous pilot carriers (CP).
In FIG. 8B, the pilot carriers are arranged also at intervals of eight carriers, and shifted in position for each symbol. These pilot carriers, therefore, are called scattering pilot carriers (SP).
The maximum delay time in the multi-path communication that can be equalized depends on the intervals of the carriers arranged, and the fading rate that can be equalized depends on the intervals of the symbols arranged. Also, the CP and the SP may be combined to improve the transmission performance.
These pilot carriers, however, with the amplitude and the phase thereof fixed, provide a signal having a predetermined repetitive period for each symbol. In the case where the guard intervals of the signal containing the pilot carriers are correlated in the receiver shown in FIG. 16, the correlation is found over the entire symbol period as well as the guard period, and therefore undesirably has an effect on the correlation result.
The problem described above is explained below specifically with reference to the OFDM signal having the CP carriers arranged for each 8 carrier samples.
In the correlation waveform (FIG. 4) obtained by the receiver shown in FIG. 16, the waveform floor (the bottom portion of the signal) actually rises due to the correlation over the entire symbol period of the CP carriers. FIG. 9 is a diagram for explaining the comparison of these guard interval correlation waveforms. The dotted line designates the guard interval correlation waveform for the received signal lacking the CP carriers shown in FIG. 4, and the solid line designates the guard interval correlation waveform for the received signal with the CP carriers inserted therein.
The ratio of floor level to peak level is 0.125 in the case where the CP carriers are inserted at intervals of eight carriers. In the case where the amplitude of the CP carrier is larger than and {fraction (4/3)} of that of the normal data carrier, the figure may reach 0.22, resulting in a higher floor level.
A higher floor of the correlation waveform and the resultant smaller level difference between peak and floor increases the chance of generating an error in the detection result of the true peak point. In the transmission path, not a small amount of noises come in. In the received signal containing noises, the probability is high that the correlation peak value of the guard intervals assumes a higher value than the original peak value. A higher floor, therefore, has the disadvantage that the peak positions are erroneously detected even with small amount of noises, and is more likely to have an adverse effect on the control operation of the sampling clock.
Also, in the case where the FFT demodulation window for the received signal is formed based on the peak positions, the likelihood of erroneous detection of peak positions leads to the disadvantage that an inter-symbol interference occurs.
As described above, even with different symbols, the CP carriers have a correlation with each other over the entire symbol period. Once multiple paths occur, therefore, as shown in FIG. 10, the correlation between the CP carriers of the main waves or between the CP carriers of the reflected waves, coupled with the correlation between a given CP carrier of the main wave and a given CP carrier of the reflected waves, generates a complicated correlation waveform.
As a result, the delay profile shown in FIG. 11 develops waveform irregularities, and the reflected waves of small level disadvantageously become unobservable.
The primary object of the invention is to provide a technique for extracting only the correlation component of the pilot carriers and a technique for extracting a correlation component of the data carriers by extracting the correlation component of the pilot carriers, in a receiving apparatus for receiving a signal of the multi-carrier modulation scheme in which pilot carriers fixed in amplitude and phase are arranged at regular intervals between the data carriers. Extraction of only the correlation component of the pilot carriers leads to various advantages in receiving the signal by the multi-carrier modulation scheme as described below.
For example, the modulation scheme of the signal sent out by a transmitter is can be identified automatically. At the transmitting end, the differential phase shift keying (hereinafter referred to as the differential scheme) or the absolute detection scheme is selectively used in accordance with the transmission path characteristics. In the differential scheme, the pilot carriers of CP and SP are not arranged in the transmission signal. In the case where the modulation scheme is changed at the transmitting end, the receiver is required to automatically follow the particular change of the modulation scheme. According to this invention, the modulation scheme can be automatically identified at the receiving end. The presence or absence of a correlation component of the pilot carrier is detected by the cross-correlating operation of the guard intervals of the received signal. In this way, it becomes possible whether the modulation scheme of the received signal is the differential scheme or the absolute detection scheme.
Further, according to this invention, only the correlation component of the data carrier can be extracted from the signal containing both the correlation component of the data carrier and the correlation component of the pilot carrier by detecting the correlation component of the pilot carrier. As a result, the disadvantages and the problems of the conventional receiver described above can be obviated. Specifically, in the correlating operation of the guard intervals with pilot carriers inserted, the adverse effect of the correlation component of the pilot carriers is removed and only the correlation of the data carriers is extracted. In this way, the performance is improved for the synchronization using the correlating operation of the guard intervals (the sampling clock frequency of the receiver is controlled in such a manner as to coincide with the sampling clock frequency of the transmission signal) or the generation of a delay profile.
In a receiving apparatus and a receiving method according to this invention, a signal modulated by the multi-carrier modulation scheme for transmitting the information code using a plurality of carriers is received, which signal contains data carriers and regularly-inserted pilot carriers fixed in amplitude and phase. The cross-correlation between the received signals with the data carriers distant from each other to such an extent as to substantially eliminate the correlation is calculated, and based on the result of the cross-correlation by the correlating operation unit, the correlation signal component associated with the pilot signals is extracted.
According to an embodiment of the invention, there are provided a receiving apparatus and a receiving method, in which the cross-correlation between the received signal and a signal obtained by delaying the received signal by a predetermined length of the symbol period or a time length corresponding to the repetitive period of the pilot carrier is calculated, and the result of this calculation is used as a first cross-correlation. Based on the first cross-correlation, the correlation component of the signal associated with the pilot carriers is extracted.
According to another embodiment of the invention, there are provided a receiving apparatus and a receiving method, in which the cross-correlation between the received signal and a signal obtained by shifting the received signal by the length of a valid symbol period is calculated, and the result of this calculation is used as a second cross-correlation. The correlation component of the signal associated with the data carrier is obtained by subtracting the first cross-correlation from the second cross-correlation.
According to still another embodiment of the invention, there are provided a receiving apparatus and a receiving method, in which the signal in the extraction range corresponding to the guard interval period is extracted from the result of subtracting the first cross-correlation from the second cross-correlation, and each time the extraction range is shifted by one sample period, the extraction process is repeated over the entire signal section for the correlating operation, and the values of the signal extracted are added. Based on the result of this addition, the difference of the symbol phase of the received signal from a predetermined reference phase is determined, and in accordance with the detected difference, the sampling clock frequency of the received signal is controlled.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.